The present invention relates to power conversion apparatus and methods, and more particularly, to AC power conversion apparatus and methods A power supply device, such as an uninterrupted power supply (UPS), may include an input rectifier that is used to generate DC voltage from an AC source (e.g., an AC utility). It is usually desirable that the rectifier circuit maintain a high waveform quality at the AC input, e.g., provide low harmonic currents and/or a power factor near unity (1). In some UPS, power factor control is achieved using a current-mode controlled pulse-width modulated (POM) rectifier, i.e., a rectifier that senses AC input current and responsively modulates the rectifier such that the AC input current waveform is substantially in phase with the AC input voltage. Such a conventional rectifier may sense the AC input voltage and appropriately scale and compensate it to generate a waveform reference for the current loop. Examples of such a control scheme are described in U.S. patent application Ser. No. 10/286,027 to Taimela, filed Nov. 1, 2002, and incorporated herein by reference in its entirety.
Such an approach can generally be effective, but the current loop may need relatively high bandwidth in order to track the AC voltage waveform when it is distorted. However, coupling of such a high bandwidth rectifier to a relatively high impedance source, such as a motor-generator set, can cause an input pole frequency of the rectifier to fall within the current loop bandwidth, which can lead to loop instability.